Indices and Indicators for Measuring Groundwater Condition and Vulnerability with Respect to Quantity and Quality in the Ruhuna Basin
Ground water is the main source of drinking water in the Ruhunu basin. Seventy percent of the population relies on ground water. As such the ground water resource is very significant in the overall management of the fresh water resources in the basin for its sustainable use. The scope of this paper is to evaluate the condition and vulnerability as a first step towards the development of management strategies for sustainable use of the fresh water resource. The UN World Water Assessment Programme (WW AP) has developed a series of indicators and indices to represent the condition and vulnerability of ground water in a region (Gangopadhyay, 2001) and these indices and indicators are used in this paper to benchmark the condition and vulnerability of this basin. Keeping in mind that these indices and indicators developed by WW AP are for a general region; they are examined here for its relevance to the Ruhuna basin and improvements deemed necessary are also suggested. In the development of values for indices and indicators for the basin, the availability, reliability and representative ness of the data significantly effects the outcome and therefore the data used for the study is also examined for its significance and improvement in the future. The sources of data used for the development of indices and indicators are those available from previous studies, explorations and borehole logs as detailed under references. As an outcome of this study this paper also discusses the direction of future studies for the more accurate development of the indices and indicators and for the development of the ground water resources of the basin in general for its sustainable use.
1. Significance of the Ground Water Resource
It is estimated that over 70% of the population of about one million inhabitants in this basin uses ground water as the source of drinking water. 70% of this population can be assumed to use ground water for purposes of bathing and washing. Figures 1 show the distribution of the sources of drinking water in the district of Monaragala in 1994 (IlCA, 2001). Various studies done on a few villages in the basin have shown that the per capita consumption of water for drinking and washing varies with socio-economic standard of the population. Assuming the average consumption per person to be 51.3 and 101.5 liters/day for drinking and washing respecti vely the current annual ground water demand can be estimated at 31 MCM.
The mean annual rainfall of the basin is 1,574 mm. Although some studies show a decline in the annual rain fall in the country over the last few years, in the long term no significant change in the annual rainfall patterns in this basin have been observed (Silva, 1984; NSF, 2000). The mean annual rainfall distribution within the basin in the short term shows large variations with some areas of the basin recording annual rainfall as low as 723 mm (De Silva, 1998). The monthly rainfall patterns have also shown considerable variation over the years. The lowest consecutive six-month rainfall of 148.4 mm was recorded in the Kirindi Oya and Malala Oya basins in 200l. These areas experienced a serious shortage of ground water during this time. Although the mean annual rainfall and the consequent recharge of aquifer within the basin is reasonably good, some
I Consultant-Engineering Solutions (Pte) Ltd., Sri Lanka.
45
areas of the basin have experienced severe water shortages of catastrophic proportions in the recent past causing iIrunense hardship to the inhabitants of these areas.
The present use of groundwater for irrigation is not very significant as discussed elsewhere under the Ruhuna Basin Case Study. The viability of the planned industrial and infrastructure development identified as essential for the socio economic sustainability of the region would depend a lot on the availability of groundwater. Therefore the availability of groundwater of acceptable quality would be a key factor for the sustenance of the population and for the socio economic stability of the region.
2. Characteristics of the Aquifers
2.1 Hydro-geology
The geological characteristics of the basin have been discussed elsewhere in this Sri Lanka Case Study. Considering the hydrogeology of the basin the ground water yielding aquifers can be identified in to two main groups, the shallow 'regolith' aquifer which is about 10m thick overlaying the basement rock and the deep fractured zones in the basement rock below 4Om. The shallow aquifer slopes gradually towards the southern coastline as shown by the contour map of the basement rock in figure 2. Therefore the shallow aquifer continuously drains out to the sea under a positive hydraulic gradient. On average at approximately 15 km inland from the coast the basement rock rises above the mean sea level. Therefore the loss of fresh groundwater to the sea is .significant and virtually uncontrollable. As such the storage capacity of the aquifer is limited. Another significant feature of this basin is the well-distributed network of streams and rivers encouraging the quick inflows to them from the surface run-off and the base flow. As such the inherent storage capacity of the aquifer is further reduced.
2.2 Groundwater Recharge
Although the annual potential evapotranspiration (PET) varies from 1400 mm in the upper reaches to 2000 rnm in the lower reaches, different studies have estimated the annual evapotranspiration by indirect methods of computation to be between 900 mm and 1200 mm. However more accurate models with data from representative regions would have to be made to make an accurate estimate. Similar studies have estimated the average annual recharge from rainfall to ground water to be 20mm to 125 mm. An average value of loomm is taken for computations in this paper. It should also be noted that a very significant recharge occurs from surface water bodies and return flows. This recharge needs to be estimated and included in the assessment of the groundwater condition indices and also be considered in the management of the groundwater resource.
2.3 Water Quality
Studies in the southern district of Hambantota in the Ruhuna basin have concluded that about 60% of the wells are contaminated with saline water (Silva, 1984). Excessive depth of dug wells and over exploitation of wells may have contributed to the high salinity. Due to geological composition the occurrence of fluoride in ground water is high (Dissanayake, 1985). Analysis of data from limited studies of tube wells in selected areas of the basin have shown that only 21 % of the wells have acceptable levels of electric conductivity and fluoride to current Sri Lanka standards for potable water (SLS 614: 1983 Parts 1 & 2). The water quality distribution of WRB tube wells is shown in figure 3. More significant is that of these wells 32% of the high yielding
46
wells with over 100 lpm have acceptable levels of electric conductivity and fluoride as shown in . in the figure 4. However, studies on shallow wells have recorded a much higher number of wells of acceptable water quality as shown in figure 5.
e under 2.4 Groundwater Yield:ructure
would Most well yield studies have been carried out on tube wells. These wells exploit the deep aquifer. later of The distribution of yield rates recorded for the 384 Water Resources Board (WRB) wells are ,e socio shown in Figure 6. Records of yield rates for the wells in the shallow aquifer are not available. Due to the wide fluctuation of the water table observed over the year, the potential yield will vary significantly with the seasons. Potential yield from the shallow aquifer could be estimated from the records of the temporal variation of the water table. However such records of not been made for this basin.
3. Data on the Groundwater Condition ka Case can be
Data collection done to exploit the ground water resource in this basin have been limited to the n thick collection of data for the purpose of finding locations where good quality groundwater is
)m. The available to meet the immediate needs of the inhabitants in the area. Aerial photographs have map of been studied to establish areas having geological characteristics most likely to have groundwater
the sea sources. Limited surveys of dug wells and information on tube wells including pumping test data oastthe obtained during their construction are available. Landsat imagery has been used to a limited ,the sea extent to <;>btain geomorphologic and geological data, lineament and bedding interpretation.
limited. Geophysical explorations have also been done in selected areas. Hydro meteorological data drivers required for water management such as the monitoring of rainfall, potential evapotranspiration,
.uch the river discharge records in a few stations, are continuously obtained on a monthly basis. Chemical
, analyses of water samples have also been done to a very limited extent. Apart from using some of this data for the location of tube wells, data have not been made available as useful information for the management or improvement of the groundwater resources of the basin. By this some data perhaps is not available for beneficial groundwater improvement programs and in other cases lost e upper or duplicated by other studies. Under an on going study (JICA, 200 I) it has been proposed to
annual , collate all available data and present them in a GIS environment for some areas of this basin.
00 mm. made to The current status of data availability is summarized in Table 1. ge from lken for Irs from 4. Groundwater Indicators and Indices for the Basin d in the nt of the The indicator and indices computed for the Ruhuna basin from limited information available this
basin is given in Table 2. Some areas of the basin undergo severe shortage of ground water during , certain periods of drought years. This is not clearly reflected in the computed indices and
therefore the indicators computed are not representative of the groundwater condition and vulnerability of these areas. Further refinement of these indices is required. This can be done by
at about improving the quality and distribution of data as well as by modifying the indices to suit the llg wells character of the aquifers in this basin. ological alysis of The benefit of a well-developed set of indices and indicators for the basin should not be under '21% of estimated, as it is an important tool to forecast the sustain ability of the groundwater resource, to i Lanka guide the development of a groundwater policy and to monitor remedial measures to sustain this ;:>fWRB resource. yielding
47
The changes to the groundwater resource and the environment as a whole occur gradually and become apparent over a long period of time, sometimes causing irreparable damage. As such early identification would be very beneficiaL The change in the forest cover of Sri Lanka from 1965 to 1992 is shown in figure 7 as an example of the gradual effects of anthropogenic and natural activity occurring in the country. The development of indicators may be able to identify such changes early. The recent shortages of water may also be due to the changes to the policy of water management. Figure 8 shows the number of abandoned ancient tanks in one part of the basin due to the emphasis on large irrigation schemes in the recent years. Some indicators are required to identify the effects and benefits of such changes. It is clear that a well-distributed system of small tanks would have contributed to a gradual recharge of the groundwater off setting the inherent weakness of regolith aquifer.
5. Conclusions & Recommendations
1. The validity of indices and indicators developed in this study are limited by the spares availability of data and its quality. Since two distinct groundwater aquifers have been identified for this basin the indices and indicators developed must reflect representative values taking in to account the overall contribution of each these aquifers. The scope of this study however did not permit a systematic allocation of weights for the significance of each aquifer to the individual indices. Since the availability of data was limited and not distributed evenly for each aquifer such a detailed study is irrelevant for this basin at this stage. However, when these indices and indicators mentioned in this paper are considered these limitations have to be kept in mind.
2. The indicator values for Groundwater Reliance and Groundwater Depletion with respect to the groundwater condition, for the Ruhuna basin presented in this paper appear to be satisfactory. However, in the recent past some areas of the basin have experienced sever water shortages. This may be due to the fact that the indicators do not reflect the salient features of this basin. The significant character of this basin is that the phreatic aquifer (regolith) on which there is a high dependence is only about 10m deep sloping continuously towards the coastal boundary of the basin. As such the temporal distribution of groundwater recharge becomes very significant. Temporal distribution of recharge is affected by the temporal distribution of rainfall and the spatial distribution of surface water bodies. The influence of the temporal distribution of the rainfall to the groundwater condition can be clearly identified. Therefore in the assessment of vulnerability indices and its value as an indicator of the influence of mitigating programs that are required for the sustainable use of the groundwater resource in this basin, the influence of the temporal distribution of groundwater recharge must be considered for this basin.
3. In order to develop more meaningful indices it is important to collate all available data to a GIS platform.
4. Data collection needs to be improved to obtain useful targeted information. For the regolith aquifer continuous data is needed, as the temporal influences are very high.
5. With respect to the availability of data, the Ruhuna basin can be considered highly vulnerable, as there is no mechanism to regulate the use of water or to control pollution. The extent of the aquifer, the source of recharge and the quantity of recharge of the deep aquifers has not been investigated. As such the extent of vulnerability is not known. Therefore target studies are required to address these issues.
6. Computer based groundwater simulation models will be an effective economical tool for both short and long-term evaluation of the regolith aquifer, for the effective planning of further studies on the groundwater resource as well as to introduce regulations for the sustainability of this groundwater resource. An effective groundwater policy would have to be implemented
48
ally and As such .ka from !nic and identify
)olicyof rt of the ltors are stributed Ff setting
Ie spares lve been sentative )e of this ! of each Istributed However, mitations
'espect to :ar to be :ed sever le salient c aquifer tinuously undwater :d by the dies. The m can be .tue as an ble use of bution of
, data to a
e regolith
lulnerable, lent of the ; not been tudies are
)1 for both of further tainability llemented
to ensure sustainability of the aquifers taking into consideration the dependence of all the rich diverse eco systems within the basin. A well-designed computer based groundwater model would be the best available tool for implementing and maintaining an effective groundwater policy.
References
Dissanayake, CoB., and Weerasooriya, S.V.R., (1985) The Hydrochemical Atlas of Sri Lanka
Gangopadhyay, Subhrendu, Indices and Indicators for Measuring Ground Water Condition and Vulnerability: Ground Water Quantity- Draft as on 2 July, 2001
Hydrology Division, Irrigation Department, Sri Lanka, Hydrological Annual 1997/98
Japan International Cooperation Agency (JICA) , Water Resources Board (WRB), Ministry of Irrigation & Water Resources Management; Pacific Consultants International, (2001) The Study on Comprehensive Groundwater Resources Development for Hambantota and Moneragala Districts in the Democratic Socialist Republic of Sri Lanka - Progress Report (1)
National Science Foundation, Colombo, Sri Lanka, Natural Resources of Sri Lanka 2000
Rajasuriyar L., 2001 - Data from tests carried out on water samples obtained from wells in Ridiyagama, Udawalawe and Suriyawewa, IWMI
Silva, K.P.L.E., (1984) Ground Water Explorations in Hambantota District Sri Lanka
Sri Lanka Standards Institution, Sri Lanka Standard 614 : 1983, Specification for Potable Water, Part 1- Physical and Chemica~ Requirements.
Acknowledgements: The data and support received from the International Water Management Institute for this study is gratefully acknowledged.
49
Figures and Tables
Table 1. Availability ofData for the Groundwater Resource ofthe Ruhuna Basin.
Topographic maps showing the stream drainage network, surface-water bodies, landforms, cultural feature, and locations of and activities related to water Geological maps of surficial deposits and bedrock
showing extent and boundaries of
Streamflow data, measurements of gain and loss of streamflow between stations Maps of the stream drainage network showing extent of normally perennial flow, normally dry channels, and
seasonal flow.
Location of recharge areas (aerial recharge from precipitation, losing streams, irrigated areas, recharge basins. and and estimates of "",,,'h,,,,,,p
50
Available. Should be implemented in GIS
GIS. Details
Not available
A vaiJable. Inadequate. Should be implemented GIS
Table J. Continued.
tnGIS
in GIS. Details
tails of well logs
Chemical Framework Geotechnical characteristics of earth materials and naturally Available. Not well defined. Should be occurring ground water in aquifers and confining units implemented in GIS Spatial distribution of water quality in aquifers, both aerially Available. Should be implemented in GIS and with depth Temporal changes 10 water quality, particular!y for Temporal information not available. contaminated or potentially vulnerable unconfined aquifers Sources and types of potential contaminants Information not available Chemical characteristics of artificially introduced waters or Information not available waste liquids Maps of land cover/land use at different scales depending on A vailable. Should be implemented in GIS study needs Stream flow quality, (water-quality sampling in space and Temporal information not available time), particularly during periods of low flow
i
I
I
lte loss
:nown
e implemented
te implemented
51
Tab
le 2
. In
dica
tors
& I
ndic
es fo
r th
e R
uhun
a B
asin
.
IND
ICA
TO
RV
AR
IAB
LE
(S)
DA
TA
V
AL
UE
IN
DE
XN
AM
E
VA
LU
E/S
UB
IN
DE
X
Con
diti
on
Ann
ual
aver
age
31
MC
M
8%
gr
ound
wat
er
Pri
mar
y/S
econ
dary
w
ithd
raw
als
Gro
und
Wat
er R
elia
nce
41
0M
CM
to
tal
wit
hdra
wal
s
Ann
ual
aver
age
Pri
mar
y/S
econ
dary
(s
urfa
ce a
nd g
roun
d
wat
er)
T
he v
alue
s as
sign
ed a
re f
or
Ann
ual
aver
age
the
shal
low
sub
sur
face
gr
ound
wat
er
6%
3.1 M
CM
aq
uife
r.
Gro
und
wat
er d
eple
tion
P
rim
ary/
Sec
onda
ry
wit
hdra
wal
s A
nnua
l av
erag
e M
ost d
ata
for
the
deep
ba
sefl
ow
55
9M
CM
fr
actu
red
zone
not
ava
ilab
le.
Hor
izon
tal
Sec
onda
ry/A
nal y
zed
Acc
ordi
ng t
o S
ilva
(W
RB
di
spla
cem
ent o
f O
vera
ll m
ore
seri
ous
sali
nity
fro
nt f
rom
a
Pri
mar
y/ A
nal y
zed
No
Dat
a 19
84)
60%
of
Ham
bant
ota
prob
lem
in c
erta
in a
reas
of
the
basi
n. (
Kir
indi
Oya
&
dist
rict
is c
over
ed b
y re
fere
nce
poin
t sa
line
gro
und
wat
er
Enc
roac
hmen
t Rat
io
Mal
ala
Oya
are
as i
n th
e D
ista
nce
of c
ontr
ol
Ham
bant
ota
Dis
tric
t)
poin
t fro
m t
he
Pri
mar
y/S
econ
dary
N
o D
ata
re
fere
nce
poin
t
Max
imum
S
ince
the
sur
face
aqu
ifer
su
bsid
ence
with
P
rim
ary/
Ana
lyze
d is
sha
llow
, la
rge
resp
ect t
o a
chos
en
Not
obs
erve
d se
ttle
men
ts d
ue t
o th
e S
ubsi
denc
e R
atio
co
nsol
idat
ion
of
the
clay
M
inim
um e
leva
tion
da
tum
la
yers
can
not
be
of
land
sur
face
with
ex
pect
ed.
Loc
al
resp
ect t
o ch
osen
P
rim
ary/
Sec
onda
ry
sett
lem
ent d
ue to
the
da
tum
co
llap
se o
f un
line
d du
g w
ells
can
occ
ur.
Tab
le 2
. C
onti
nu
ed
Vul
nera
bili
ty
Hyd
raul
ic
Pri
mar
y/S
econ
dary
l A
(1 t
o 10
m2l~ay)/O.5
Gro
und
Wat
er
leve
l ~iffusi
vity
na
l)'z
ed
say
2 to
20
rn2/
day
decl
ine
Het
erog
enei
ty
Pri
mar
y/S
econ
dary
l A
high
H
igh
vuln
erab
ilit
y na
lyze
d ~~~
Ann
ual
Rec
harg
e P
rim
ary/
Sec
onda
ry/A
5
59
MC
M
naly
zed
Vol
ume
of
Aqu
ifer
P
rim
ary/
Sec
onda
ry/A
2
79
5M
CM
H
igh
vuln
erab
ilit
y na
lyze
d H
ighl
y va
riab
le
Hyd
raul
ic G
radi
ent
Pri
mar
y/S
econ
dary
/A
0.00
14
Acc
ordi
ng to
Sil
va (
WR
B
Sal
t Wat
er i
ntru
sion
na
lyze
d 19
84)
60%
of
Ham
bant
ota
Per
mea
bili
ty
Pri
mar
y/S
econ
dary
/A
0.00
1 m
/day
di
stri
ct is
cov
ered
by
~al)'zed
sali
ne g
roun
d w
ater
E
ffec
tive
ness
of
Pri
mar
y/S
econ
dary
N
ot k
now
n V
ulne
rabi
lity
Hig
h h)
'dra
ulic
bar
rier
s ----
Str
atif
icat
ion
of
Pri
mar
y/S
econ
dary
S
urfa
ce s
andy
cla
y A
quif
er
~-~
Tot
al T
hick
ness
of
Pri
mar
y/S
econ
dary
3
t05
m
(not
obs
erve
d)
Lan
d su
bsid
ence
aq
uita
rds
Vul
nera
bili
ty l
ow
Com
pres
sibi
lity
of
Pri
mar
y/S
econ
dary
/A
Ver
y lo
w
aqui
tard
s na
lyze
d ~-~
Hyd
raul
ic
Pri
mar
y/S
econ
dary
/A
(l t
o 10
m2/
day)
/0.5
U
sual
ly r
ecov
ers
wit
hin
1 di
ffus
ivit
y na
lyze
d sa
y 2
to 2
0 rn
2/da
y to
3 y
ears
S
tabi
liza
tion
tim
e C
hara
cter
isti
c P
rim
ary/
Sec
onda
ry
len
gth
of b
asin
B
asin
Con
stan
t ---
Figure 1. Distribution of the source ofdrinking water in the Monaragala District in 1994. (J1CA, 2001 ).
River\Tank\St ream\ others
18%
Tube Well 7%
ped Supply DugW 12%63%
54
Figure 2. Topography of the Basement Rock in the Hambantota District in m.s.l.m. (Silva, 1984).
.., ." .. .. c
..• 1 I 1: .II~ ~ • Iii i;; • -'Ii
-i)... i j• 1- '5
E e81 A
•l: m• \
0
Jli'o+'
~ , .. ;,..... • ' .
.'
o
..
.• "
!
55
Figure 3. Distribution ofElectrical Conductivity of384 Tube Wells of WRB.
EC> 1500 mS/cm
39%
No Data
8%EC < 1500 mS/cm & F > 1.5 mg/I
32%
EC < 1500 mS/cm & F < 1.5 mg/l
21%
Figure 4: Distribution ofElectrical Conductivity of384 Tube Wells of WRB with yields over 100 lpm.
EC> 1500 mS/cm
26%
EC < 1500 mS/cm & F < 1.5 mgll
32%
No Data 18%
EC < 1500 mS/cm & F > 1.5 mg/I
24%
56
Figure 5. Distribution of Electrical Conductivity, Fluoride & Nitrate from well survey of66 dug wells at Ridiyagama, Suriyawewa and Udawalawe (IWM I, 2001).
EC > 1500 mSlc:m, F> 1.5 mgll & N03EC < 1500 mSlcm,
> 10F> 1.5 mgtl & N03 > 10 mgll
21%
EC < 1500 mSlc:m, EC < 1500 mSlcm, F < 1.5 mgll & N03 F < 1.5 mgll & N03 < 10 mgll
> 10 mgll 63% 2%
,100
Figure 6. Distribution of Well Yields in the WRB tube wells.
Yield> 100 Ipm 16%
100 Ipm>Yield > 50 Ipm
13% Yield < 50 Ipm 71%
57
Figure 7. Sri Lanka's Forest Cover in 1965 (left) and 1992 (right).
58
59
Figure 8. Abandoned Ancient Tanks (Silva, 1984).
g ~ 0< 0:
: ~ . . acr • •
0.. (;:x: .. 0 aJ
... ".,... if <:
E., "0
" <> tl e.,
if>..;:
; \~:e;!
--....--~ .